The present invention relates to toners for use in developing electrostatic images by dry development electrophotography, electrostatic printing and the like.
Dry development electrophotography utilizes finely divided dry toners or developers. Dry toners typically contain natural or synthetic resins having a coloring agent, such as carbon black, dispersed therein. Dry developers typically are two component systems which contain dry toners combined with magnetic carrier particles. Dry development processes include, for example, cascade, hair brushing, magnetic brushing, impression and powder cloud processes. All of these processes involve developing a formed electrostatic latent image using the toner.
After the image is developed by the toner, the image is "fixed." Fixation can be effected by melting the resin particles in the toner thereby fusing them together. Melt fixing can be accomplished by applying heat and/or a solvent to the toner image generally after the image has been transferred to sheet of paper. In the heat fixing method, heat is applied to the sheet of paper by means of heated fuser rollers. The heated fuser rollers apply both heat and pressure to the sheet, thereby permanently fixing the toner image onto the sheet. The surfaces of the fuser rollers are typically made from, or coated with, a non-stick material to prevent the toner from preferentially sticking to the surface of the rollers. Residue on the fuser rollers can be deposited on sheets which subsequently pass through the rollers, which results in poor image quality.
One method which has been used to prevent or minimize this problem is to coat the surface of the fuser roller with a non-stick material, such as a fluorocarbon resin and/or to apply a silicone oil to the roller so that a thin coating of the oil is maintained on the roller. This method suffers from several drawbacks: for example, the heat from the fuser rollers can cause odors to emanate from the liquid and/or the roller coating, and the use of an the silicone liquid requires a complex apparatus for metering the liquid to the rollers.
Another method which has been used to reduce sticking is adding a wax, such as paraffin or polyolefin wax, to the toner itself. In theory, the wax component should be sufficiently incompatible with the toner resin so that it does not mix with the tone resin. This is to ensure that when the paper imprinted with the toner image passes through the hot fuser rollers, the wax can rise to the surface of the toner thereby forming a waxy coating which prevents the toner from sticking to the rollers. However, paraffin waxes and polyolefin waxes (e.g., polyethylene or polypropylene) may adhere to the roller and/or paper surface, thereby reducing the quality of the image. In some toner compositions, the polyolefin wax is sufficiently compatible with the toner resin that it associates with the resin and does not tend to rise to the surface of the toner particles to prevent the toner from sticking to the fuser rollers. The result is the occurrence of offset and the resulting poor quality images.
It is an object of the present invention to provide a dry toner which is free of polyethylene, polypropylene or paraffin wax, and which does not require the use of oils or other liquids for use in developing electrostatic images.